The present application is directed to a method for injection molding using liquid vinyl polymers.
Liquid vinyl polymers have commonly been employed in dip molding, casting, coating, and rotational molding processes in the form of plastisols. The process of dip molding has been used commercially for over sixty years to produce articles of various shapes and configurations. In a dip molding process, a heated mold form or mandrel is dipped into a plastisol bath whereupon a layer of gelled plastisol forms about the mandrel. The coated material is removed from the bath and the coated layer cured, resulting in a molded article corresponding in configuration to the configuration of the mandrel employed. For instance, when a cylindrical mandrel (or pin) is employed, a tubular article can be produced. Beverage containers, bicycle handles, gloves and tubular hose bend restrictors are but a few of the products that are presently produced by dip molding by use of varying types of mandrels.
The production of molded articles by dip molding presents some problems, such as providing a molded article having well-defined edges or end portions. For example, it is known that tubular articles frequently have end portions that curl around the end of the mandrel, with the undesirable result that the ends of the tube curl into the interior of the tube. Such a result can be overcome can cutting or trimming the offending edge. However, such an additional step is time consuming and constitutes an additional expense (both with respect to labor as well as wasted raw material).
Some attempts have been made to address such problems, as noted in U.S. Pat. Nos. 2,058,552 and 3,005,237, by way of example.
However, while the use of dip molding processes has produced relatively satisfactory products, and has done so for many years, the dip molding process has not been found to be satisfactory to the extent that well-defined products are desired to be produced.
Injection molding processes have been found to avoid such problems due to the greater spatial definition or geometry afforded by the interior of the mold. However, as the use of liquid plastisol-type materials has been limited to dip molding processes, the use of such materials in injection molding has heretofore not been believed appropriate, especially due to the use of a screw conveyor in injection molding. The screw conveyor has previously been thought generally best adapted for use with solid polymer particles which are fed to the injector housing to be melted and forced into the mold by the screw conveyor and which require significant work to undertake a phase change (solid to liquid) prior to being molded.
The use of a liquid polymer in injection molding is described by Rogers U.S. Pat. No. 4,783,292. The Rogers patent teaches the addition of pressurized gas bubbles into a pre-heated liquid polymer, with the liquid polymer subsequently being forced into a mold cavity whereby the bubbles expand to form a foamed plastic article.
Zelasko U.S. Pat. No. 4,600,150 discloses a spraying system wherein a material to be sprayed is heated prior to being fed to a screw conveyor means, whereupon the heated material is passed to a spray means.
The use of injection molding is also known for LIM silicone (Liquid Injection Molding Silicone) which is a two part liquid silicone (thermoset) material that uses a pumping system and static mixer. The components mix prior to being introduced into the injection barrel and the liquid maintained at room temperature until being heated in the mold where the silicone material is cured.
However, the prior art fails to satisfactorily address the disadvantages noted above.